Intrinsically safe voltage clamping device

ABSTRACT

An intrinsically safe voltage clamping device includes a regulated rail, a ground rail, and a shunt regulator assembly. The shunt regulator assembly is coupled to both the regulated rail and the ground rail and includes one or more regulating components. The shunt regulator assembly is configured to clamp a voltage applied across the regulated rail and the ground rail to a safety clamp voltage value. The intrinsically safe voltage clamping device also includes a power-sensing component configured to cause one or more limiting components to reduce a power dissipated in the respective regulating components without raising the clamp voltage.

RELATED APPLICATIONS

This application claims the benefit of priority to, U.S. ProvisionalPatent Application No. 61/896,475 filed Oct. 28, 2013, and entitled“VOLTAGE CLAMP DEVICE FOR INTRINSIC SAFETY,” which is herebyincorporated by reference herein, in its entirety, for all purposes.

FIELD OF THE DISCLOSURE

The present disclosure is directed to an intrinsically safe device and,more particularly, to an intrinsically safe voltage clamping device withthermal and/or power limiting.

BACKGROUND

Some industrial processes, such as those in the petroleum industry,require devices to operate in hazardous atmospheres or other dangerousconditions. Such devices are often governed by an “intrinsically safe”standard, such as the ISA-60079-11 standard, specifying certainconditions for devices in hazardous atmospheres. The conditions seek toeither limit the amount of energy stored in device circuitry (e.g., bylimiting voltages to capacitors or currents to inductors) or limit thedischarge of accumulated energy (e.g., by restricting the spacing ofcomponents) such that a discharge of energy will not cause an ignition.Further, in the case of the ISA-60079-11 standard, a circuit must remainsafe during normal operation of the circuit even with a certain numberof faults.

In some standards, an individual apparatus (or device) is treateddifferently than an assembly of components that is part of a largerapparatus. In the ISA-60079-11 standard, for example, a shunt safetyassembly manufactured as an individual apparatus must adhere to adifferent section of the standard, as compared with a shunt safetyassembly that is part of a larger apparatus. The differing requirementsfor an individual apparatus can allow manufacturers to produce deviceswith advantageous properties, such as physically compact packages.However, such self-contained voltage clamping devices can also sufferfrom failures related to overheating, making them impractical in manyapplications.

SUMMARY

An intrinsically safe voltage clamping device comprises a regulatedrail, a ground rail, and a shunt regulator assembly. The shunt regulatorassembly is coupled to both the regulated rail and the ground rail andincludes one or more regulating components. Also, the shunt regulatorassembly is configured to clamp a voltage applied across the regulatedrail and the ground rail to a safety clamp voltage value. Theintrinsically safe voltage clamping device also includes a thermallyactivated component configured to, when the temperature of at least oneof the regulating components exceeds a threshold value, cause one ormore limiting components to reduce a power dissipated in the at leastone of the regulating components.

In another embodiment, a process control device comprises a devicecomponent having first and second terminals, the device componentstoring energy when a voltage is applied across the first and secondterminals of the device component. The process control device alsoincludes two or more voltage clamping devices electrically coupled tothe device component. Each voltage clamping device is disposed inparallel with the other of the voltage clamping devices, and eachvoltage clamping device is configured to clamp the voltage appliedacross the device component to a safety clamp voltage. Further, eachvoltage clamping device comprises a shunt regulator assembly includingone or more regulating components, and a thermally activated componentconfigured to, when the temperature of at least one of the regulatingcomponents exceeds a threshold value, cause one or more limitingcomponents to reduce a power dissipated in the at least one of theregulating components.

In another embodiment an intrinsically safe voltage clamping devicecomprises a regulated rail, a ground rail, and a shunt regulatorassembly. The shunt regulator assembly is coupled to both the regulatedrail and the ground rail and includes one or more regulating components.Also, the shunt regulator assembly is configured to clamp a voltageapplied across the regulated rail and the ground rail to a safety clampvoltage value. The intrinsically safe voltage clamping device alsoincludes a current-sensing resistor configured to, when the currentthrough the resistor exceeds a threshold value, cause one or morelimiting components to reduce a power dissipated in the at least one ofthe regulating components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system in which primary andredundant voltage clamping devices are implemented.

FIG. 2A illustrates an example circuit which may be packaged as anintrinsically safe voltage clamping device and implemented as one of thevoltage clamping devices illustrated in FIG. 1.

FIG. 2B illustrates another example circuit which may be packaged as anintrinsically safe voltage clamping device and implemented as one of thevoltage clamping devices illustrated in FIG. 1.

FIG. 3A-3C are block diagrams of example device packages in whichintrinsically safe voltage clamping devices, such as the circuitillustrated in FIG. 2, can be packaged.

DETAILED DESCRIPTION

The present disclosure is directed to an intrinsically safe voltageclamping device and, specifically, to preventing thermal damage tovoltage regulating device components in a self-contained device packagevia thermal- and/or power-limiting components. In particular, a voltageclamping device according to the present disclosure meets the functionalrequirements of a shunt voltage regulator while, at the same time,allowing for a physically compact and intrinsically safe packaging. Inembodiments, thermally limiting components are utilized to automaticallylower the clamp voltage of the device and reduce power dissipation inone or more regulating components. As a result, the limiting componentsmay protect the voltage regulating components of the device againstdamage related to overheating while maintaining the safety-criticalmaximum clamp voltage.

Referring now to FIG. 1, an example system 100 constructed in accordancewith one embodiment of the present disclosure includes a process controldevice 106 and a power supply 108. Primary and redundant voltageclamping devices 102 a and 102 b may limit the voltage applied across adevice component 110 to a respective safety clamp voltage, in animplementation. By including two voltage clamping devices 102 a and 102b, the system 100 may adhere to an intrinsically safe (IS) standardrequiring continued voltage clamping upon failure of one of the voltageclamping devices 102 a and 102 b. An example voltage clamping device isdiscussed in more detail with reference to FIGS. 2A and 2B.

While depicted in FIG. 1 as being inside the process control device 106,the primary and redundant voltage clamping devices 102 a and 102 b maybe internal or external to the process control device. The voltageclamping devices 102 a and 102 b may be modular devices which can beremovably coupled to the process control device 106, or the voltageclamping devices 102 a and 102 b may be stand-alone devices which may beelectrically coupled to the process control device via any combinationof electrical leads. In general, the voltage clamping devices 102 a and102 b may be coupled to or part of the process control device 106 viaany suitable internal or external electrical connections, terminals,etc.

The process control device 106 may include a switch, transmitter,thermocouple, solenoid valve, etc., and, in particular, the processcontrol device 106 may include the device component 110. The devicecomponent 110 may be any type of circuit component or assembly ofcomponents that stores energy when a voltage is applied across thedevice component 110. For example, the device component 110 may includeone or more capacitors or inductors. Although only one device component110 is illustrated in FIG. 1, it is clear that a process control devicemay have any number of components capable of storing energy.

The process control device 106 may be part of a manufacturing plant, oilor gas extraction structure, refinery, HVAC (heating, ventilation, andair conditioning) system, etc. in which the process control device 102 ais exposed to a hazardous environment, such as an environment withhazardous gases, chemicals, vapors, dusts, fibers, etc. As such, theprocess control device 106 may be an intrinsically safe process controldevice 106, or the process control device 106 may be made intrinsicallysafe in combination with the voltage clamping devices 102 a and 102 b.

The example power supply 108 may power the process control device andmay be operatively connected to both the process control device 106 andthe voltage clamping devices 102 a and 102 b, where the process controldevice 106 and the voltage clamping devices 102 a and 102 b areconnected in parallel (e.g., via two terminals of the process controldevice 106 and two leads of the voltage clamping devices 102 a and 102b). The power supply 108 provides power to various components and may,in some cases, provide operating voltages for other circuits orcomponents. For example, the power source 108 may provide outputpositive and negative voltages that are, in turn, applied to the railsof the voltage clamping devices 102 a and 102 b and the device component110. The positive and negative voltages are denoted in FIG. 1 as +V and−V, respectively, but it is understood that a positive voltage may beapplied to one of the voltage clamping devices 102 a and 102 b and thedevice component 110 and the other of the rails may be grounded.

The power supply 108 may be coupled to a mains power source, forexample, or the power supply 108 may be coupled to battery power source.Also, the power supply 108 may, in some cases, transform a power signal(e.g., 24V) to particular voltages (e.g., ±3.3V, ±10V), where theparticular voltages can be applied across rails of the voltage clampingdevices 102 a and 102 b and the device component 110.

In some implementations, the voltage clamping devices 102 a and 102 binclude: (i) regulating components 112 a and 112 b, respectively, suchas components that are part of a shunt voltage regulating assembly(transistors, amplifiers, voltage references, etc.); (ii) power-sensingcomponents 114 a and 114 b, respectively, such as thermistors, othertemperature sensors, current sensors, etc.; and (iii) one or morelimiting components 116 a and 116 b, respectively, such as diodes,transistors, etc., configured to selectively reduce power dissipation inthe regulating components 112 a and 112 b based on activation of thepower-sensing components 114 a and 114 b. Additionally, though describedhere as “power-sensing,” the components 114 a and 114 b may, instead,control the limiting components 116 a and 116 b according to anotherparameter value, such as a parameter value related to or relatable tothe temperature of the regulating components 112 a and 112 b, as will bedescribed below with respect to FIGS. 2A and 2B.

The regulating components 112 a and 112 b may include any components ofrespective shunt voltage regulator assemblies in the voltage clampingdevices 102 a and 102 b. The regulating component 112 a may include, forexample, a transistor, amplifier, and/or voltage reference devicedisposed between a regulated rail and ground rail of the voltageclamping device 102 a. In some cases, a failure (e.g., due tooverheating) of the regulating component 112 a may cause the voltageclamping device 102 a to lose the ability to clamp a voltage (e.g.,applied across the device component 110) to a safety clamp voltage.

To prevent failure of the regulating components 112 a and 112 b due tooverheating, the regulating components 112 a and 112 b may be coupled tothe power-sensing components 114 a and 114 b (as in FIG. 2A). Thecoupling between one of the regulating components 112 a and 112 b andthe respective power-sensing component 114 a or 114 b may includethermal bonding between the components. For example, the thermal bondingmay include a thermal pad or thermal transfer grease to assist in thetransfer of heat from the one of the regulating components 112 a and 112b to the respective power-sensing component 114 a or 114 b.

By allowing heat transfer from one of the regulating components 112 aand 112 b to a respective power-sensing component 114 a and 114 b, thepower-sensing component 114 a or 114 b may be activated (e.g., at acertain threshold temperature). When activated, the power-sensingcomponents 114 a and 114 b may activate respective limiting components116 a and 116 b. The limiting components 116 a and 116 b may include anysuitable circuit components (e.g., diodes, transistors, and resistors)electrically coupled to shunt voltage regulating assemblies of thevoltage clamping devices 102 a and 102 b, such that the limitingcomponents 116 a and 116 b reduce power dissipation in the regulatingcomponents 112 a and 112 b. By reducing the power dissipated in theregulating components 112 a and 112 b, the limiting components 116 a and116 b may reduce the temperature of the regulating components 112 a and112 b and prevent overheating.

In addition to being activated by the power-sensing components 114 a and114 b, the limiting components 116 a and 116 b may be de-activated whenthe power-sensing components 114 a and 114 b detect a sufficientdecrease in the temperature of the regulating components 112 a and 112b. That is, after a temperature of the regulating components 112 a and112 b is reduced (e.g., below a threshold), the correspondingpower-sensing component 114 a and 114 b and limiting components 116 aand 116 b may be de-activated.

In other embodiments, the components 114 a and 114 b are not thermallycoupled to the regulating components 112 a and 112 b, and insteadregulate the temperature of the regulating components 112 a and 112 baccording to the power dissipated in the components 112 a and 112 b. Inone embodiment, for example, the components 114 a and 114 b may sense atransistor junction voltage of a transistor in the regulating components112 a and 112 b. When the components 114 a and 114 b detect a decreasein the transistor junction voltage, the components 114 a and 114 b cancause the limiting components 116 a and 116 b to decrease the powerdissipated by the regulating components 112 a and 112 b.

In another embodiment, the components 114 a and 114 b are configured tosense voltage across and current through the regulating components 112 aand 112 b, respectively, and to calculate the power being dissipated inthe regulating components 112 a and 112 b. When the power exceeds athreshold value, the components 114 a and 114 b may cause the limitingcomponents 116 a and 116 b, respectively, to limit the voltage (e.g., bydecreasing the clamp voltage) and/or current, thereby decreasing thepower dissipated in the components 112 a and 112 b and, accordingly, thetemperature increase associated with that power dissipation.

In any event, the thermal limiting of the regulating components 112 aand 112 b may facilitate the components of the voltage clamping devices102 a and 102 b being packaged in a compact and self-contained devicepackage. This compact device package may allow a use of the voltageclamping devices 102 a and 102 b in applications where physical spaceutilized by the devices 102 a and 102 b is a concern. Moreover, thevoltage clamping devices 102 a and 102 b may comply with individualapparatus requirements of an intrinsically safe standard, whichrequirements allow for less redundancy in an implementation of thevoltage clamping devices 102 a and 102 b than would otherwise berequired in a more complex apparatus.

FIG. 2A illustrates an example circuit 200 which may be packaged as anintrinsically safe voltage clamping device, such as one of the voltageclamping devices 102 a and 102 b. Although, the example circuit 200 isdescribed below with reference to certain components, it is clear thatany suitable values and types of components may be utilized to providethe voltage clamping and thermal limiting functionality of the circuit200.

The example circuit 200 includes a shunt regulator assembly 201configured to clamp a voltage applied across a regulated rail 202 and aground rail 204 to a safety clamp voltage. The shunt regulator assemblymay include one or more regulating components such as one or moreresistors 206, an amplifier 208, a reference voltage 210 coupled to anon-inverting terminal of the amplifier 208, and a transistor 212coupled to a voltage output terminal of the amplifier 208. In theexample circuit 200, the amplifier 208 drives the transistor 212according to the difference between the reference voltage 210 and thefeedback voltage from the shunted regulated rail 202. As such, most ofthe power dissipated in the example circuit 200 will be dissipated inthe transistor 212, and the transistor 212 may, in some cases, increasein temperature.

Such an increase in the temperature of the transistor 212, or in one ormore other regulating components of the shunt regulator assembly 201,may activate the thermally activated component 214 which is thermallycoupled to the transistor 212 (as indicated by the box 216). Activationof the thermally activated component 214 may correspond to a variety ofchanges in the thermally activated component 214. For example, thethermally activated component 214 may be an NTC thermistor which dropsin resistance upon an increase in the temperature of the transistor 212.It is understood, however, that the thermally activated component 214may include any suitable temperature sensor, such as a PTC thermistor,integrated circuit, etc. Alternatively, the circuit 200 may utilize anamplifier to compare an actual temperature of the transistor 212 to atargeted maximum temperature or threshold.

Upon activation, the thermally activated component 214 causes one ormore limiting components, such as a transistor 218, to reduce the powerdissipated in regulating components, such as the transistor 212. In oneexample scenario in which an NTC thermistor is implemented as thethermally activated component 214, the thermistor 214, acting as part ofa temperature-dependent voltage divider between the regulated rail andground rail, varies the voltage at the base of the transistor 218, andmay decrease in resistance (i.e., increase the voltage at the base ofthe transistor 218) enough to “turn on” the transistor 218. That is, thevoltage drop across the thermally activated component 214 may decreaseso as to increase the voltage difference between the base and theemitter of the transistor 218.

Such an increase may cause the transistor 212 to be turned on “harder”(e.g., the transistor current will increase) than dictated by theamplifier 208. As a result, the clamped voltage between the regulatedrail 202 and the ground rail 204 will drop allowing a reduction in boththe power dissipated in the transistor 212 and the temperature of thetransistor 212.

The example circuit 200 may further include various other components(e.g., resistors and diodes) to adjust current, voltage, etc. values orratings. In one case, the circuit 200 may be configured for applicationsin which 4-20 mA intrinsically safe circuits are used. As such, thecurrent typically flowing in the example circuit 200 may be less than 25mA, even though the circuit 200 may be rated for a maximum current of130 mA. During normal operation, the circuit 200 may be able to operatewith up to 25 mA being shunted. However, the circuit 200 may reduce ashunt voltage (e.g., via limiting components) to protect regulatingcomponents in the abnormal condition of up to 130 mA of current.Alternatively, the circuit 200 may be rated for up to 195 mA to meetoverrating safety factors required by IS standards.

While depicted in FIG. 2A as bipolar junction transistors, thetransistors 212 and 218 need not be BJTs and, instead, the circuit 200may be designed with metal-oxide-semiconductor field effect transistor(MOSFET) technology, as will be readily appreciated.

In one scenario, a 6V shunt regulator shunting 25 mA would dissipate 150mW. As such, the example circuit 200 may be able to dissipate thisamount of power without going into a thermally limited mode (e.g.,activating the thermally activated component 214). If the maximumtemperature of the die of the transistor 212 is 150° C. and the deviceis operating in an 85° C. environment, a thermal resistance from die toambient of 233° C./W would be adequate to provide a 30° C. operatingmargin. Further, if the circuit 200 is to be rated for 195 mA, thecircuit 200 need only reduce the safety clamp voltage (e.g., vialimiting components, such as a transistor 216) below 1.43V to keep thedie temperature below 150° C. in such abnormal operating conditions.

FIG. 2B depicts another embodiment in which current sensing is used tocontrol the thermal load of the regulating component. In FIG. 2B, anexample circuit 220 functions much the same as the circuit in FIG. 2A.That is, during normal operation of the circuit 220, the amplifier 208similarly drives a regulating transistor 222 according to the differencebetween the reference voltage and the feedback voltage from theregulated rail 202. Most of the power dissipated in the circuit 220 willbe dissipated in the transistor 222. If the current being shunted by thetransistor 222 is low enough (e.g., below 25 mA), a transistor 224 willbe in cut-off. If the current being hunted by the transistor 224 througha resistor 226 is sufficient to drop enough voltage across the resistor226 to turn on the transistor 224, then the transistor 224 will turn onthe transistor 222, decreasing the clamp voltage of the circuit 220. Thecurrent-driven voltage limiting action of the transistor 224 willoverride the normal shunt voltage control (provided by the combinationof the amplifier 208 and the transistor 222) until the current beingshunted falls below the level where the transistor 224 is being turnedon. For example, the circuit 220 is capable of shunting 20 mA at thedesigned clamp voltage of 6 V. If the current supplied to the circuit220 were 200 mA, however, the voltage of the regulated rail 202 wouldfall below 1.4 V. The effect of the current-driven clamp voltagereduction is to limit the power dissipated in the transistor 222.

In other embodiments, the voltage across the resistor 226 may beamplified to more precisely control the current at which the clampvoltage is pulled down. In still other embodiments, a multiplier circuitcan be implemented to pull down the voltage only as much as necessary tolimit the power. However, these improvements are not strictly necessaryto accomplish the intended thermal limitation function.

FIGS. 3A-3C illustrate example configurations of device packages inwhich intrinsically safe voltage clamping devices may be packaged. Forexample, the voltage clamping devices 102 a and/or 102 b or the circuits200 or 220 may be packaged in a manner similar to that illustrated inFIGS. 3A-3C.

FIG. 3A is a block diagram of one example device package 300 in which anintrinsically safe voltage clamping device may be packaged. The devicepackage 300 includes a set 302 of components configured to providevoltage clamping functionality and thermal limiting functionality, suchas the sets of components (e.g., the circuit 200) illustrated in FIG. 2Aor 2B. A power supply, such as the power supply 108, may apply a voltageto the components 302 via two leads 310 and 312, and the two leads 310and 312 may, in this example package 300, be the only two leads externalto the device package 300. The device package 300 may, in some cases, bemanufactured or assembled in a manner that adheres to intrinsically safestandards. The device package 300 may be, for example, dust-tight and/ormay meet clearance distance and mechanical requirements for a specificapplication. In addition, the materials used to construct the devicepackage 300 may include materials selected based on electricalcharacteristics. Specifically, the materials of the device package 300may fulfill certain electrostatic conditions to prevent an accumulationof static charge.

A two-lead device package, such as the device package 300, may also beheld to different intrinsic safety requirements as compared with avoltage clamping circuit that is part of a larger device. Further, toadhere to an intrinsic safety standard, the device package 300 may beconnected in parallel with a redundant voltage clamping device via thetwo leads 310 and 312. The redundant voltage clamping device may includesimilar components to the device package 300, but, in general, mayinclude any suitable assembly of components, leads, and connections.

FIG. 3B is a block diagram of another example device package 320 inwhich an intrinsically safe voltage clamping device may be packaged. Aswith the device package 300, the device package 320 includes a set 322of components configured to provide voltage clamping functionality andthermal limiting functionality. In this case, however, a power supplymay apply a voltage to the components 322 via three or more leads 324and 326 (three leads, four leads, five leads, etc.).

The three or more leads 324 and 326 of the device package 320 may beshorted together or otherwise combined or connected such that only twoleads 328 and 330 are connected to the components 322 of the devicepackage 320. Although FIG. 3B illustrated the leads 324 and 326 beingconnected inside of the device package 320, it is understood that thethree or more leads 324 and 326 may be combined or connected external orinternal to the device package 320.

A device package, such as the device package 320, may be subject tospecific sections of an intrinsic safety standard due to the inclusionof more than two electronic leads. As such, the leads 324 and 326 may beselectively shorted together internally or externally withoutinterference with the voltage clamping functionality of the devicepackage 320, thereby maintaining adherence to a standard. Further, thedevice package 320 may be redundantly combined with other voltageclamping devices to maintain intrinsic safety.

FIG. 3C is a block diagram of yet another example device package 360 inwhich an intrinsically safe voltage clamping device may be packaged. Thedevice package 360 also includes a set 362 of components configured toprovide voltage clamping functionality and thermal limitingfunctionality. However, in addition to two leads 364 and 366, the devicepackage 360 includes one or more additional leads 368. The one or moreadditional leads 368 may include unused pins of an integrated circuit(IC), leads of a power transistor package, etc.

Although the device package 360 includes the additional leads 368, theadditional leads 368 may not be electrically connected to any othercircuit component, (as indicated by an “X” in FIG. 3D). In this way, thedevice package 360 may ensure that the additional leads 368 may notinterfere with voltage clamping functionality of the device package 360and, in some cases, ensure that the device package 360 adheres tocertain intrinsically safe standards. For example, a certainintrinsically safe standard may require an integrated circuit to remainsafe with any combination of its leads electrically shorted.

Although FIGS. 3A-3C illustrate a certain number of distinct components,leads, and connection types, it is clear that a device package mayinclude any number and combination of components, leads, and connectiontypes. For example, a device package may include two, three, five, etc.leads coupled to both a regulated and ground rail, or a device packagemay include zero, one, two, etc. electronic leads in addition to twoprimary electronic leads (e.g., coupled to a power supply).

What is claimed is:
 1. An intrinsically safe voltage clamping devicecomprising: a regulated rail; a ground rail; a shunt regulator assemblycoupled to both the regulated rail and the ground rail and including oneor more regulating components, the shunt regulator assembly configuredto clamp a voltage applied across the regulated rail and the ground railto a safety clamp voltage value; and a power-sensing componentcomprising: (1) a thermally activated component configured to, when atemperature of at least one of the regulating components exceeds athreshold value, cause one or more limiting components to reduce a powerdissipated in the at least one of the regulating components, or (2) acurrent-sensing resistor configured to, when the current through theresistor exceeds a threshold value, cause one or more limitingcomponents to reduce a power dissipated in the at least one of theregulating components, wherein the regulated rail, the ground rail, theshunt regulator assembly, the power-sensing component, and the limitingcomponents are packaged together in a two-lead device package, andwherein the device package is coupled, via the two leads, to at leastone redundant device package configured to clamp the voltage appliedacross the redundant device package to a redundant safety clamp voltagevalue.
 2. An intrinsically safe voltage clamping device comprising: aregulated rail; a ground rail; a shunt regulator assembly coupled toboth the regulated rail and the ground rail and including one or moreregulating components, the shunt regulator assembly configured to clampa voltage applied across the regulated rail and the ground rail to asafety clamp voltage value; and a power-sensing component comprising:(1) a thermally activated component configured to, when a temperature ofat least one of the regulating components exceeds a threshold value,cause one or more limiting components to reduce a power dissipated inthe at least one of the regulating components, or (2) a current-sensingresistor configured to, when the current through the resistor exceeds athreshold value, cause one or more limiting components to reduce a powerdissipated in the at least one of the regulating components, wherein theregulated rail, the ground rail, the shunt regulator assembly, thepower-sensing component, and the limiting components are packagedtogether in a device package having more than two leads, and wherein twoleads of the device package are electrically coupled, respectively, tothe regulated rail and the ground rail, and wherein additional leads areconfigured such that any combination of shorts between the leads doesnot interfere with the voltage clamping function of the intrinsicallysafe voltage clamping device.
 3. The intrinsically safe voltage clampingdevice of claim 2, wherein the device package is a power transistorpackage.
 4. The intrinsically safe voltage clamping device of claim 1,wherein the power-sensing component is the thermally activated componentand wherein the thermally activated component is thermally coupled tothe one or more regulating components by physical proximity.
 5. Theintrinsically safe voltage clamping device of claim 1, wherein thepower-sensing component is the thermally activated component and whereinthe thermally activated component is thermally coupled to the one ormore regulating components by a thermal bonding agent.
 6. Theintrinsically safe voltage clamping device of claim 1, wherein the oneor more regulating components includes a first transistor and whereinthe one or more limiting components includes a second transistor.
 7. Theintrinsically safe voltage clamping device of claim 6, wherein reducingthe power dissipated in the at least one of the regulating componentsincludes activating the second transistor to clamp the voltage appliedacross the regulated rail and the ground rail to a second voltagesmaller in magnitude than the safety clamp voltage value.
 8. Theintrinsically safe voltage clamping device of claim 6, wherein thepower-sensing component is the current-sensing resistor and wherein theresistor is configured to cause the second transistor to put the firsttransistor into saturation mode when the current sensed at the resistorexceeds the threshold value.
 9. A process control device comprising: adevice component having first and second terminals, the device componentstoring energy when a voltage is applied across the first and secondterminals of the device component; two or more voltage clamping deviceselectrically coupled to the device component, each voltage clampingdevice disposed in parallel with the other of the voltage clampingdevices, and each voltage clamping device configured to clamp thevoltage applied across the device component to a safety clamp voltage,each voltage clamping device comprising: a shunt regulator assemblyincluding one or more regulating components, and a power-sensingcomponent configured to, when a temperature or current exceeds athreshold value, cause one or more limiting components to reduce a powerdissipated in the at least one of the regulating components, wherein theone or more regulating components includes a first transistor andwherein the one or more limiting components includes a secondtransistor, and wherein reducing the power dissipated in the at leastone of the regulating components includes activating the secondtransistor to clamp the voltage applied across the device component to asecond voltage smaller in magnitude than the safety clamp voltage value.10. The process control device of claim 9, wherein each of the two ormore voltage clamping devices are packaged in a device package havingtwo external electronic leads.
 11. The process control device of claim9, wherein each of the two or more voltage clamping devices are packagedin a device package having two primary external electronic leads and oneor more secondary external electronic leads.
 12. The process controldevice of claim 11, wherein the secondary electronic leads of the devicepackage are not electrically connected to anything inside the devicepackage.
 13. The process control device of claim 9, wherein thepower-sensing component is a thermally-activated component and whereinthe thermally activated component includes a temperature sensor and anamplifier.
 14. The process control device of claim 9, wherein thepower-sensing component is a thermally-activated component and whereinthe thermally activated component includes a thermistor.
 15. The processcontrol device of claim 9, wherein the power-sensing component is acurrent-sensing resistor, and wherein the current sensing resistor isconfigured to cause the one or more limiting components to reduce apower dissipated in the one or more regulating components when thecurrent through the resistor exceeds a threshold value.
 16. Theintrinsically safe voltage clamping device of claim 2, wherein the oneor more regulating components includes a first transistor and whereinthe one or more limiting components includes a second transistor.
 17. Anintrinsically safe voltage clamping device comprising: a regulated rail;a ground rail; a shunt regulator assembly coupled to both the regulatedrail and the ground rail and including one or more regulatingcomponents, the shunt regulator assembly configured to clamp a voltageapplied across the regulated rail and the ground rail to a safety clampvoltage value; and a power-sensing component comprising: (1) a thermallyactivated component configured to, when a temperature of at least one ofthe regulating components exceeds a threshold value, cause one or morelimiting components to reduce a power dissipated in the at least one ofthe regulating components, or (2) a current-sensing resistor configuredto, when the current through the resistor exceeds a threshold value,cause one or more limiting components to reduce a power dissipated inthe at least one of the regulating components, wherein the one or moreregulating components includes a first transistor and wherein the one ormore limiting components includes a second transistor, and whereinreducing the power dissipated in the at least one of the regulatingcomponents includes activating the second transistor to clamp thevoltage applied across the regulated rail and the ground rail to asecond voltage smaller in magnitude than the safety clamp voltage value.18. The intrinsically safe voltage clamping device of claim 2, whereinthe one or more regulating components includes a first transistor andwherein the one or more limiting components includes a secondtransistor, and wherein the power-sensing component is thecurrent-sensing resistor and wherein the resistor is configured to causethe second transistor to put the first transistor into saturation modewhen the current sensed at the resistor exceeds the threshold value.